1. Field of the Invention
The present invention relates to an integrated circuit, for example a microcomputer, and more particularly, it relates to an integrated circuit utilizing different power supply voltages for circuits constructing an input/output part and an internal circuit.
2. Description of the Background Art
FIG. 8 is a block diagram showing the structure of a conventional microcomputer 10. The microcomputer 10 comprises a CPU (central processing unit) 1, a RAM (random access memory) 2, a ROM (read only memory) 3 and an input/output pad part (input/output interface) 50, which are connected with each other through a data bus DTB and an address bus ADB.
The structure of the input/output pad part 50 is now described with reference to FIG. 9. The input/output pad part 50 comprises an input protection circuit 51 which is connected to an external terminal OT, an output buffer 52 which is connected to the input protection circuit 51, a TTL input detection circuit 53, a Schmidt type input detection circuit 54 and an input/output control circuit 4. A signal supplied to the external terminal OT passes through the input protection circuit 51 and is divided into three paths, if the same is a normal signal. Namely, the signal is divided into the three paths to be supplied to the input/output control circuit 4 as a CMOS input detection signal CM and supplied to the TTL input detection circuit 53 and the Schmidt type input detection circuit 54 respectively.
The input protection circuit 51 is adapted to protect the circuits in the input/output pad part 50 when the external terminal OT is supplied with a nonstandard signal of a high voltage. The operating voltage for the input protection circuit 51 is set at 5 V, for example. If a signal of a voltage exceeding 5 V is inputted, the input protection circuit 51 reduces the voltage to 5 V for outputting this signal. If a signal of a voltage smaller than 0 V is inputted, on the other hand, the input protection circuit 51 increases the voltage to 0 V. Therefore, a 5 V-system signal inputted in the input/output pad part 50 passes through the input protection circuit 51, while a signal outputted from the output buffer 52, which is a 5 V-system signal, passes through the input protection circuit 51 to be outputted from the external terminal OT.
The TTL input detection circuit 53, which is adapted to determine the voltage level of an input signal with the TTL standard, is employed for transferring/receiving signals in the TTL system for supplying its output to the input/output control circuit 4 as a TTL input detection signal TL. On the other hand, a recent microcomputer which is constructed by the CMOS technique requires no specific detection circuit in case of transferring signals in the CMOS system.
The Schmidt type input detection circuit 54, which is adapted to remove noise or the like included in the input signal, is constructed by a Schmidt circuit or the like for supplying its output to the input/output control circuit 4 as a Schmidt type input detection signal ST.
Either the TTL input detection circuit 53 or the Schmidt type input detection circuit 54 is employed in response to the use of the external terminal OT. For example, the TTL input detection circuit 53 is employed when the external terminal OT is used as a memory interface, while the Schmidt type input detection circuit 54 is employed when the external terminal OT is used as a serial communication interface.
A control system for an electronic appliance, particularly a system employing a microcomputer, is generally constructed by semiconductor devices operating with a power supply voltage of 5 V. While the power supply voltage is set at 5 V on the basis of historical factors, it is convenient for transferring signals between semiconductor devices if the microcomputer 10 and all semiconductor devices mounted thereon operate with the same power supply voltage.
However, the specification of such a microcomputer is increasingly sophisticated, and a higher operating frequency is required, for example. If the operating frequency is improved, however, power consumption is disadvantageously increased in proportion to this. Following recent refinement resulting from high integration, high electric fields are caused in the semiconductor devices with the power supply voltage of 5 V, leading to a problem such as characteristic deterioration caused by a hot carrier phenomenon. Such a problem can be solved by lowering the power supply voltage for the microcomputer, and different power supply voltages may be employed for circuits constructing an input/output part and an internal circuit such as a logic part or a memory part. For example, Japanese Patent Laying-Open Gazette No. 4-336812 (1992) or 3-145744 (1991) describes such a structure.
However, this structure leads to a problem in the conformity with the remaining semiconductor devices constructing the system. If the aforementioned microcomputer 10 employs a power supply voltage of 3.3 V, for example, for the internal circuits such as the CPU 1, the RAM 2 and the ROM 3 while driving a semiconductor device externally mounted on the microcomputer 10 with the power supply voltage of 5 V, for example, the input/output pad part 50 must employ the power supply voltage of 5 V to be connected with this semiconductor device. However, the internal circuits operate with the power supply voltage of 3.3 V, and hence signals cannot be directly transferred between the input/output pad part 50 and the internal circuits.
In a general microcomputer, further, different output characteristics are required to a so referred to general port such as a data input/output port, an analog input port or a reset input port and the so-referred to as system port for connecting the microcomputer with an external device such as an externally mounted memory for constructing a system. Namely, the general port must have an output characteristic which is as loose as possible, while the system port must have a steep output characteristic. An operation of varying the output characteristics with terminals is referred to as slew rate control.